The present invention relates to a method and apparatus for reducing spray penetration length for diesel engines, and more particularly to a method and apparatus that uses supercavitation so as to allow highly atomized sprays to be employed for small engine, high-pressure diesel injection systems.
Over the past two decades, advances in high-pressure, electronically-controlled diesel fuel injection technology have had a significant impact on diesel engine efficiency, exhaust emissions, and engine noise. These advances have been primarily limited to larger engines in the output power range of 40 hp or greater. This has been due to inherent technical limitations with scaling to smaller engines. Along with the development of electronically controlled fuel injection, researchers and manufacturers have made use of increasing fuel pressures as a method to produce highly atomized sprays and deliver fuel to the cylinder quickly. Highly atomized spray patterns provide for improved engine efficiency and the ability to deliver fuel to the cylinder quickly which is required by the high rotational speeds of high-power-density engines. The conventional approach has been to enhance these features by using high fuel pressures (up to 2,500 bar) but this has been found to result in long fuel jet penetration lengths, something which is not desirable for a small engine injection system. Long fuel jet penetration lengths in small engine piston bowls/cylinders lead to wetting the piston bowl wall and cylinder wall which can reduce fuel vaporization rates, increase emissions, increase ignition delay, and wash lubricants from the cylinder wall decreasing durability/performance. The prior art recognizes that to provide a high-pressure injector for small engines much less than 40 hp range requires that the fuel spray penetration length be significantly decreased but up until now has not found any practical way of doing so.
It is generally recognized that the presence of bubbly cavitation in diesel injectors improves the quality of atomization. For example, U.S. Pat. Nos. 7,798,130 and 7,533,655 discuss how a sonic nozzle aids in dispersing the fuel into the air with cavitation bubbles to produce a fine atomization. But it is also well known that the unsteady and erratic nature of typical cavitation can lead to cycle-to-cycle variations in spray penetration and nozzle balance, and therefore cavitation is something that is normally avoided to the extent possible in injector nozzles. For example, U.S. Pat. No. 7,850,099 discloses a method to reduce the risk of cavitation. Likewise, U.S. Pat. No. 7,841,544 discloses an injector configuration that provides a minimum of flow cavitation, and U.S. Pat. No. 7,578,450 B2 discusses how his design has the benefit of decreasing cavitation effects within the tip. U.S. Pat. Nos. 7,740,187 and 7,793,862 discuss the promotion of cavitation within the fuel injector control valve portion, but not the injection tip.                Although supercavitation has never been suggested for a fuel injector nozzle, supercavitation in mico-channels is well documented in the literature. For example, Schnieder, B., Kosar, A., and Peles, Y. “Hydrodynamic Cavitation and Boiling in Refrigerant (R-123) Flow Inside Microchannels” (International Journal of Heat and Mass Transfer, Vol. 50, 2007) has demonstrated the following pattern for supercavitating flows in micro-channels:        A liquid jet is created immediately following the nozzle orifice and extends 20 to 25 orifice diameters downstream of the orifice;        A transition region follows after the liquid jet region and this transition region's length is determined by the cavitation number of the flow;        A wavy annular region that only occurs with low cavitation numbers and is characterized by a vapor core surrounded by a liquid annulus; and        A bubbly region in which, as the static pressure continues to rise, the vapor core collapses leaving only a few remaining bubbles.        
We have discovered, however, that with the proper type of cavitation, namely supercavitation, the unsteady and erratic performance associated with typical cavitation can be avoided, the improved atomization (known to exist with all types of cavitation) can be provided and the liquid spray penetration length shortened. The reduction in spray penetration length allows smaller diesel engines to be built with effective high-pressure, low-emissions atomizing injectors.
More specifically, we have discovered an injector nozzle profile where the fuel exits the nozzle in the above-mentioned wavy annular region (which only occurs with low cavitation numbers), but tailoring the flow morphology in which the bubbly flow regime exists at the exit and thereby to provide the desired reduction in fuel penetration length.
One of the main parameters that control this flow pattern morphology is the cavitation number which is defined as:
                    σ        =                                            P              e                        -                          P              V                                                          1              2                        ⁢            ρ            ⁢                                                  ⁢                          μ              2                                                          (        1        )            where Pe is the exit pressure, Pv is the vapor pressure of the fluid, ρ is the liquid-phase density, and u is the liquid velocity at the inlet restrictor. FIG. 1 shows the effect of the cavitation number on the types of flow regimes that are present for water. An additional experiment was conducted with R-123 and the trends are similar, however the actual cavitation index at the transitions did change, however, demonstrating that the results obtained are a function of the fluid used.
Prior research into improved fuel injectors has studied the effects of cavitation on the discharge coefficient, the area blockage of the injector hole, the momentum and mass fluxes emanating from the injection holes, the quality of the spray, and erosion problems inside of the injector. However, these prior studies appear to have been conducted with either bubbly cavitation or string cavitation, which is resultant of flow vortices that are created as the liquid passes through the injection channel [see, e.g., Schnieder, B., Kosar, A., and Peles, Y. “Hydrodynamic Cavitation and Boiling in Refrigerant (R-123) Flow Inside Microchannels” International Journal of Heat and Mass Transfer, Vol. 50, 2007; Payri, F., Arregle, J., Lopez, J., and Hermens, S. “Effect of Cavitation on the Nozzle Outlet Flow, Spray and Flame Formation in a Diesel Engine” SAE Paper No. 2006-01-1391, Society of Automotive Engineers, Warrendale, Pa., 2006; Gavaises, M., Papoulias, D., Andriotis, A., Giannadakis, E., and Theodorakakos, A. “Link Between Cavitation Development and Erosion Damage in Diesel Injector Nozzles” SAE Paper No. 2007-01-0246, Society of Automotive Engineers, Warrendale, Pa., 2007; and Giannadakis, E., Papoulias, D., Gavaises, M., Arcoumanis, C., Soteriou, C., and Tang, W. “Evaluation of the Predictive Capability of Diesel Nozzle Cavitation Models” SAE Paper No. 2007-01-0245, Society of Automotive Engineers, Warrendale, Pa., 2007.
Unlike these known configurations, our approach uses a supercavitating inverse annular flow inside the injector for improved atomization, reduced penetration lengths, and improved fuel distribution.
The atomization process for high pressure injection systems (Pinj>1200 bar) is typically divided into two different processes, primary atomization and secondary atomization. Primary atomization is a result of surface waves generated by turbulence or cavitation upstream in the nozzle which are amplified by aerodynamic forces until fracture of the liquid jet emanating from the injection hole as discussed in Rotondi, R., Bella, G., Grimaldi, C., and Postrioti, L. “Atmoization of high-Pressure Diesel Spray: Experimental Validation of a New Breakup Model” SAE Paper No. 2001-01-1070, Society of Automotive Engineers, Warrendale, Pa., 2001. The size of the droplet at fracture is dependent on the initial diameter of the liquid jet, the surface tension at the fuel/air interface, relative velocities, and the viscosity of the air [Id.]. In a supercavitating spray that utilizes a wavy annular flow morphology there are two surfaces that the aerodynamic forces act on; the outside and inside of the annular jet. A non-cavitating spray has only one surface that the aerodynamic forces act on; the outside of the round jet. Therefore, the atomization for a round liquid jet and a wavy annular flow are drastically different.
Secondary atomization, as its name suggests, is the process of initial droplets further breaking down. This process is unaffected by the origin of the droplets, so the utilization of supercavitating flow has minimal impact.
Other phenomenon that impact the design of the proposed supercavitating nozzle is that once supercavitation occurs, the mass flux through a given orifice is choked. However, with an increase in upstream pressure, the momentum flux will continue to increase [see, e.g., Desantes, J., Payri, R., Salvador, F., and Gimeno, J. “Measurements of Spray Momentum for the Study of Cavitation in Diesel Injection Nozzles” SAE Paper No. 2003-01-0703, Society of Automotive Engineers, Warrendale, Pa., 2003]. This is directly related to the penetration length along with droplet size.
Therefore, an object of the present invention is to provide an injection nozzle profile induces supercavitating flows with the wavy annular flow path at the minimum upstream pressure to gain the benefits of the flow morphology and quick fuel delivery but avoid excessive momentum flux that would induce a longer penetration length.